Amplifier



Dec. 24, 1940.

c. L. RICHARDS ET AL AMPLIFIER Filed Aug. 3, 1938 hu-M luvsN'r-oni CLAUDE LANGDON R/CHA 0S FEEDER/K KERKHOF Patented Dec. 24, 1940 UNITED" STATES PATENT OFFICE" AMPLIFIER Claude Langdon Richards and Frederik Kerkhof, Eindhoven, Netherlands, assignors, by mesne' assignments, to Radio Corporation of America, New York, N. Y., a corporation of Delaware Application August a, 1938, scrim No. 222,810 In the Netherlands August 25, 1937 4 Claims.

Such a circuit arrangement is preferably utilized in a television receiver since in this case it is frequently impossible to provide in that porl tion of the cathode lead which is common to the grid circuit and to the anode circuit, a resistance component which determines the average brightr ness of the received image and of which it is undesired to be sup-plied to the grid of the high 25 or intermediate frequency amplifying tubes.

According to the invention, an improvement in the circuit arrangement with which the invention is concerned, is obtained by providing filters which suppress the high or intermediate frequen- 30 cy oscillations in the conductors by means of which the grid and the cathode of the tube or tubes to be controlled are connected to the potentiometer resistance from which the variable grid bias is taken.

35 The conductors by means of which the grid and the cathode are connected to the potentiometer resistance preferably have connected in them screened resistances the screening of which is earthed.

40 The invention will be explained more fully with reference to the accompanying drawing wherein:

Fig. 1 represents a circuit arrangement with which the invention is concerned.

45 Fig. 2 represents one mode of realization of a high or intermediate frequency amplifier according to the invention. v

The circuit arrangement shown in Fig. 1 com prises a high frequency amplifying tube l to the input circuit 2 of which the high frequency oscillations received in an aerial 3 are supplied and the output circuit of which includes an impedance 4 from which the oscillations amplified by the tube 1 are taken. The anode voltage ofv the tube l is determined by the drop of voltage in a resistance 5 across which occurs part of the voltage of a source of voltage 6, which part is determined by the ratio between the resistances 5 and 1. The grid bias of the tube l is taken from a potentiometer resistance 8 which is connected in j 5 parallel with that part of the source of voltage -6 which is determined by the value of the resistance 1. Furthermore, decoupling condensers 9 and l 0 are provided which bridge the resistances 8 and 1 respectively for the oscillations to be 10 amplified while a condenser II is connected in parallel with the source of voltage 6.

"The above described circuit arrangement has the drawback that for the oscillations to be amplified the condensersli: and I0 do not form complete short circuit but that due to the dielectric losses in these condensers and to the selfinductance of the connecting wires, high frequency voltages are set up between the points denoted by l2. and I3 and the earth. These voltages may be capacitativelytransmitted to the aerial 3, which gives rise to an undesired feedback. This drawback arises in particular if the ampliher is constructed as shown in Fig. 2 wherein the volume control of the apparatus can be efiected from a distance.

In the embodiment shown in Fig. 2 the ends of the resistance 8 are connected by means of conductors l4 and Hi to the amplifier proper, the

point l2 being connected through the intermediary of a conductor H to a slide contact l6 on the resistance 8. Since the conductors l5 and I1 are capacitatively coupled with the aerial 3, there exists with this circuit arrangement a very great possibility that a high frequency voltage set up between the points I2 and l 3 and the earth is supplied via the aerial 3 to the grid of the tube I.

According to the invention, in order to remove this drawback the conductors by means of which 40 the grid and the cathode of the tube l are connected to the potentiometer resistance 8 have connected to them screen resistances l8 and I9 of which the screening member is earthed. These resistances'act as filters which suppress the high frequency voltages set up between the points I2 and I3 and the earth, owing to which it is achieved that between the terminal points and the slide contact l6 of the resistance 8 and the earth there is no high frequency voltage. The

resistance I9 is provided in the conductor connecting the cathode to the potentiometer resistance 8 in such manner that the anode direct current does not pass through this resistance. A further advantage of the circuit arrange- 1. An amplifier comprising a thermionic tube having an anode, control electrode and. cathode, a tuned resonant circuit connected between the control electrode and cathode, a potentiometer having two end terminals and a tapped terminal included in the connection of the tuned resonant circuit to the cathode, a first shielded resistor' connected from the tuned resonant circuit to said tapped terminal of the potentiometer, a second shielded resistor connected to the cathode and one of said end terminals of the potentiometer, a source of voltage for supplying energy to the anode of the tube, a voltage divider connected across the source of voltage, and connections from said second shielded resistor to a portion of the voltage divider.

2. A radio frequency amplifier comprising a thermionic tube having an anode, control electrode and cathode, a source of voltage for supplying said another with power, volume control means comprising a. potentiometer connected in parallel with at least a portion of said source of voltage, and low pass filters connected between said potentiometer and said control electrode and between'said potentiometer and said cathode for suppressing radio frequency components.

3. A radio frequency amplifier as claimed in claim 2 wherein said low pass filters comprise shielded resistances, the shields of said resistances being grounded.

4. A radio frequency amplifier comprising a thermionic tube having an anode, control electrode and cathode, a tuned resonant circuit connected between the control electrode and cathode, a source of voltage for supplying said anode with power, volume control means comtween said potentiometer and said cathode for suppressing radio frequency components.

. CLAUDE LANGDON RICHARDS.

FREDERIK KERKHOF. 

